The Pet Parent Guide to Heartworm Disease

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Heartworm Disease and your pet

30-Second Summary

  • Heartworm disease is caused by a parasite called Dirofilaria immitis. In its mature, reproductive form, this foot-long worm normally exists in and around the heart, lungs and associated blood vessels of affected pets.
  • A dog (or cat) may be infected with heartworms and show no clinical signs. If clinical signs do transpire, the disease is likely well advanced and will be more difficult to treat.
  • When an infected mosquito bites another dog, the infective larvae are dumped onto the surface of the animal’s skin. They then proceed to enter the new host through the mosquito’s bite wound.
  • Our feline friends are atypical hosts for heartworms, as most worms do not survive to the adult stage.
  • Heartworm disease often goes undetected in cats, so it’s important to test periodically.
  • The medication used to treat heartworm infections in dogs cannot be used with cats. Therefore, prevention is the only way of protecting your cat from the harmful effects of heartworm disease.
  • The 5 Most Frequently Asked Heartworm Disease Questions: Answered

When it comes to keeping your pet parasite-free, the least likely condition to infect your pet – but potentially the most damaging – is Heartworm disease. As Part 3 of the Unholy Pet Parasite Trinity, we answer all the questions you were afraid to ask.

Part 1:   Ticks and Canine Lyme Disease
Part 2:   Meet the Enemy: Fleas

Mosquitos are the worst. Go ahead, name an organism that has resulted in the deaths of more people over human history. They are among the world’s deadliest vectors for disease, including Zika, West Nile, Dengue, Yellow Fever, Malaria and….Heartworm.

Heartworm disease occurs largely in dogs, but incidences can occur in our feline friends, too. The dog is a natural host for heartworms, which means that heartworms that live inside a dog can mature into adults, mate and produce offspring.

The disease is caused by a parasite called Dirofilaria immitis. In its mature, reproductive form, this foot-long worm normally exists in and around the heart, lungs and associated blood vessels of affected pets.

If left untreated, heartworm can cause severe lung disease, heart failure, damage to other organs and sometimes, death.

The number of dogs that test positive for heartworm in North America has been steadily rising, despite the efforts of the veterinary community to increase pet parent awareness. Projections indicate that with a warming climate, these numbers will only continue to rise as the disease continues it’s northerly expansion from the southern US into Canada.

What are Heartworms?

As the name implies, Heartworm is a worm-like parasite that can grow up to 8-12 inches (!) at maturity. Once infected by a wayward mosquito, your pet becomes a ‘carrier’ or reservoir of infection.

Infectious heartworm larvae take about six to seven months to make their way to the heart, mature into adults and begin to produce new offspring – called microfilariae.

Adult worms tend to migrate and gather around the heart and the arteries that supply the lungs, while the microfilariae circulate throughout the bloodstream before reaching adulthood.

A dog (or cat) may be infected with heartworms and show no clinical signs. If clinical signs do transpire, the disease is likely well advanced and will be more difficult to treat.

How is Heartworm Disease Spread?

The mosquito is the central, dreadful actor who plays a critical role in the life cycle of heartworms. They are truly the bane of our existence.

As mentioned above, adult female heartworms reside in infected mammals – dog, fox, coyote, or wolf – and (re)produce microscopic baby worms called microfilaria. These offspring circulate in the bloodstream of infected animals as they mature.

When a mosquito bites and takes a blood lunch from an infected animal, it picks up the microfilariae (plural). Over the period of 10 to 14 days, they then develop and mature into “infective stage” larvae.

When the infected mosquito bites another dog, or any other animal including cats, the infective larvae are dumped onto the surface of the animal’s skin. It then proceeds to enter the new host through the mosquito’s bite wound.

Once they inhabit the new host, it takes nearly 6 months for the larvae to develop into sexually mature adult heartworms. In adulthood, heartworms can live for 5 – 7 years in dogs and up to 2 – 3 years in cats.

Because of their longevity and heartiness, each mosquito season can have a cumulative effect and increase the number of worms in an infected pet.

Signs of Heartworm Disease in Dogs

Detecting heartworm disease early on can be a challenge, as many dogs exhibit few symptoms or none at all. This is why prevention is key – the longer the infection persists, the more likely symptoms will develop.

As for outward signs of the disease, dogs severely infected with heartworms, active dogs or dogs with other underlying health conditions are most likely to display distinct clinical symptoms.

Signs of heartworm disease in your dog may include:

  • Unwillingness to exercise
  • Reduced appetite
  • A mild, but persistent cough
  • Tiredness after moderate activity
  • Weight loss

As the disease advances, dogs may develop heart, liver or kidney failure and a swollen abdomen due to excess fluids.

If there are very large aggregations of heartworms, dogs can develop an abrupt blockage of blood flow within the heart. This is called Caval Syndrome and is evident by the start of heavy, laboured breathing, pale gums and dark, bloody urine. This is the worst-case scenario – few dogs survive without immediate surgical removal of the heartworm blockage.

Heartworm in Cats? 

Our feline friends are atypical hosts for heartworms, as most worms do not survive to the adult stage. If heartworms reach the adult stage in cats, they typically have just one to three worms. Dogs can have hundreds of mature heartworms.

Heartworm disease often goes undetected in cats, so it’s important to test periodically – especially if your cat is allowed outdoors. The medication used to treat heartworm infections in dogs cannot be used in cats. Therefore, prevention is the only way of protecting your cat from the harmful effects of heartworm disease.

Heartworm Disease Symptoms in Cats

Signs of Heartworm disease in cats can be subtle or glaringly obvious.

Symptoms may include:

  • Suppressed appetite
  • Coughing or hacking
  • Asthma-like attacks
  • Weight loss
  • Sporadic vomiting

As for the glaringly obvious, an affected cat may struggle to walk, experience fainting or seizures, or suffer from fluid buildup in their belly. In extreme cases, the first sign is the last sign – sudden collapse or sudden death.

How Often Should I Test My Dog for Heartworm?

Ideally, all dogs should be tested annually for heartworm infection, especially in high-risk areas. An antigen test is easily administered during any routine visit for preventive care.

American Heartworm Society Guidelines on Testing and Timing:

  • Puppies under 7 months of age can be begin on heartworm treatments without a heartworm test, as a positive test for infection takes up to 6 months to be detected.
  • Testing should begin 6 months after your initial visit, and then again 6 months later. Yearly testing after that ensure they are heartworm-free.
  • Dogs older than 7 months and not on a preventive treatment plan need to be blood-tested for antigens before starting heartworm prevention. It’s recommended they be tested 6 months and 12 months later and annually after that.

Annual testing is important – even if your dog is on heartworm prevention year-round – to ensure that the prevention program is working. Heartworm medications are highly effective – but not quite 100%. It’s possible that dogs can still become infected. Miss a dose or give it late could open a window to infection.

Our Cabbagetown Care Wellness plans include Heartworm and Tick blood screens on an annual basis to keep these parasites at bay.

What if my dog tests positive for Heartworm?

A Heartworm diagnosis is bad news. The good news is that the majority of infected dogs can be successfully treated. The treatment goal is straightforward: stabilize your dog if showing signs of the disease, and then kill all adult and undeveloped worms while keeping side effects to a minimum.

What to expect if your dog tests positive:

Diagnosis Confirmation: Once a dog tests positive on an antigen test, the diagnosis may be confirmed through additional testing, such as x-rays or an ultrasound exam. Treatment for Heartworm is expensive and complex – additional testing measures by our veterinary team are needed as to absolutely ensure treatment is required.

Exercise Limits: If your dog is used to being active, this requirement may be hard to manage. Normal physical activities must be restricted as soon as the diagnosis is confirmed, as physical exertion increases the rate at which the heartworms cause damage in the heart and lungs. Simply put, the more severe the symptoms, the less activity your dog should have.

Stabilizing the Disease: Before actual heartworm treatment can begin, your dog’s condition may need to be stabilized with appropriate therapy. In severe cases of heartworm disease, the process can take several months.

Treatment Protocols: Once our veterinarians have determined your dog is stable enough for treatment, she will recommend a treatment plan, usually involving several steps to recovery.

The American Heartworm Society has guidelines for developing this plan of attack. Dogs with no signs or mild signs of heartworm disease have a high success rate with treatment.

Severe disease can also be treated, but the possibility of complications is increased. The severity of heartworm disease does not always correlate with the severity of symptoms, and dogs with many worms may have few or no symptoms early in the course of the disease.

What if my cat tests positive for Heartworm?

Cats are not ideal hosts for heartworms. However, the risk of infection still exists. Some infections resolve on their own, but can leave lasting respiratory system damage.

Diagnosis. Difficulty: High. The severity of heartworm disease in dogs is proportionate to the number of worms – just one or two worms can make a cat quite sick. Detection can be complex, usually requiring a physical exam, an X-ray and/or ultrasound and a complete blood screen.

Treatment. There is no approved drug therapy for heartworm infection in cats – drugs used to treat dogs is not safe for their feline counterparts. Great veterinary care from the Cabbagetown Pet Clinic is the treatment. The goal is to stabilize your cat and establish a long-term management plan. If mild symptoms persist, small doses of prednisolone may be administered to help reduce inflammation.

Prevention. If your cat is allowed to explore outdoors, it’s highly recommended that you provide them with monthly heartworm preventives. Preventatives for indoor cats are not mission-critical.

How can I protect my dog from Heartworm disease?

Prevention: A preventative treatment plan is, without doubt, THE best deterrent against Heartworm disease. Couple this with our Cabbagetown Care Wellness Plans and all your bases are covered. Treatments after the fact are available, but may be associated with increased health risks and tend to be expensive. Simple oral medications, such as Simparica Trio – typically administered monthly – have been shown to be very effective for the prevention of heartworm disease, as well as providing protection against ticks and fleas.

Avoidance: By reducing walking or exercising during peak mosquito periods – typically at dawn and dusk – you can help lessen, but not eliminate, the risk of mosquito bites. Note: indoor pets are not immune to heartworm disease, as mosquitos can fly into a home or apartment through open doors or windows.

Eliminate: Reduce the amount of standing water on your property, especially in rural areas of the province – mosquitos love and thrive that type of environment.

The Top 5 Most Frequently Asked Heartworm Disease Questions

Q: Can my dog die from Heartworm Disease?

A: Yes, but not likely if treated.

Heartworm disease is multifaceted and can distress many vital organs. As a result, the undesirable outcomes of this infection can differ significantly among dogs.

Adult worms – left untreated – cause inflammation of blood vessels which can result in severe heart, liver or kidney failure to the point of no return. Prevention is key.

Q: Do I need a prescription for my pet’s heartworm preventative medication? If so, why?

A: The short answer is yes. Heartworm preventives – such as Simparica Trio – must be prescribed by a veterinarian. Before writing a prescription for a heartworm preventive, our veterinarians perform a 4DX blood test to check for antigens to ensure your pet doesn’t already have adult heartworms.

Why? Although rare, prescribing preventatives to a pet that already has heartworm has been known to lead to severe reactions that could be harmful or fatal.

Q: I live in Canada. Does my pet need year-round heartworm protection?

A: In the United States, the American Heartworm Society recommends a year-round prevention program. In Canada, we follow their recommendations. Diagnoses of heartworms have been found in almost every northern state and, by extension, Canada.

Mosquito species thrive in warmer climates, but are relentlessly adapting to cooler climates as well. Climate change is a factor adding to this northerly creep.

Q: Can I get Heartworm Disease from my dog?

A: You can relax. No amount of petting – or any forms of pet contact – will result in transmission. The parasite is spread ONLY though the bite of a mosquito – it’s the only known vector for Heartworm disease acquisition. It’s a very precise parasite that affects dogs, cats and other mammals, but it’s extremely rare in humans.

Q: Can my dog get a vaccine for Heartworm Disease?

A: No. Currently, there is no vaccine available for the prevention of heartworm disease in dogs or cats.

Heartworm disease can only be prevented through the regular use of preventive medications prescribed by your veterinarian. These medications offer the added benefit of preventing other parasites – namely, ticks and fleas – as well.

Left untreated, Heartworm disease is a severe, progressive illness. Early detection is critical, as this gives your pet the best chance of recovery.

The best treatment for your pet is always year-round, all-in prevention – Simparica Trio covers all the bases.

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